Sorbent for removing nitrogen oxides, sulfur oxides and hydrogen sulfide from gas streams

ABSTRACT

Novel sorbents comprising (a) an alumina substrate having a pore volume between 0.4 and 0.8 cc/g, and (b) an alkali or alkaline earth component, for example, sodium carbonate, wherein the amount of the alkali or alkaline earth component is between 50 and 400 μg per m 2  of the substrate. The sorbents of the present invention are outstandingly effective for the removal of nitrogen oxides, sulfur oxides and hydrogen sulfide from waste gas streams.

This is a continuation of application Ser. No. 659,996, filed Oct. 12,1984, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to sorbents and processes for removingpollutants from gas streams using such sorbents. More particularly, thesorbents of the present invention are resistant to physical degradationwhich results from recurring adsorption and regeneration. Morespecifically, the invention is directed to removing nitrogen oxides,sulfur oxides and hydrogen sulfide from gas streams.

The nitrogen oxides which are pollutants are nitric oxide (NO) andnitrogen dioxide (NO₂ or N₂ O₄). The relatively inert nitric oxide isoften only difficultly removed, relative to NO₂. The lower oxide ofnitrogen, N₂ O (nitrous oxide), is not considered a pollutant at thelevels usually found in ambient air, or as usually discharged fromeffluent sources. Nitrous oxide, however, degrades (decomposes) in theatmosphere to produce nitric oxide and thus eventually becomes apolluting component.

Sulfur oxides considered to be pollutants are sulfur dioxide and sulfurtrioxide.

Particularly obnoxious sources of nitrogen and sulfur oxide pollutantsare power plant stack gases, automobile exhaust gases, heating plantstack gases, and various industrial process effluents such as smeltingoperations and nitric and sulfuric acid plants.

Power plant emissions represent an especially formidable source ofnitrogen oxides and sulfur oxides, by virtue of the very large tonnageof these pollutants in such emissions discharged into the atmosphereannually. Moreover, because of the low concentration of the pollutantsin such emissions, typically 0.05% or less for nitrogen oxides and 0.3%or less for sulfur dioxide, their removal is difficult because verylarge volumes of gas must be treated.

Hydrogen sulfide is a pollutant in the effluents of the followingoperations: coal gasification, coal liquefaction, oil shale processing,tar sands processing, petroleum processing and geothermal energyutilization.

Of the few practical systems which have hitherto been proposed for theremoval of nitrogen oxides from power plant flue gases, all have certaindisadvantages. One such process entails scrubbing the gas with a slurryof magnesium hydroxide or carbonate; the slurry is regenerated bytreatment with ammonia. This process, however, produces by-productammonium nitrate which is difficult to dispose of, and also requirescooling and reheating of the flue gas stream.

Processes for the removal of nitrogen oxides from gases using varioussorbents are discussed in the following: U.S. Pat. No. 2,684,283 to Ogg,Jr. et al (sorbent: mass of ferric oxide and sodium oxide); U.S. Pat.No. 3,382,033 to Kitagawa (sorbent: porous carrier impregnated withFeSO₄ +H₂ SO₄, FeSO₄, FeSO₄.(NH)₄ SO₄, PdSO₄, KMnO₄, KMnO₄ +H₂ SO₄,KClO₃, NaClO+NaOH, NaClO₂ +NaOH, Na₂ MoO₄, K₂ S₂ O₃, Na₂ S₂ O₃ +NaOH,NaHPO₄, Na₂ O₂, As₂ O₂ +NaOH, CuCl₂, or ICI₃ +NaOH); U.S. Pat. No.3,498,743 to Kyllonen (use of a bed of finely divided solid sodiumcarbonate); and U.S. Pat. No. 3,864,450 to Takeyama et al (use of acatalyst consisting essentially of carbon impregnated with sodium orpotassium hydroxide).

Various methods have been proposed for the removal of sulfur dioxidefrom power plant flue gases, but all of these have disadvantages. Forexample, wet scrubbing systems based on aqueous alkaline materials, suchas solutions of sodium carbonate or sodium sulfite, or slurries ofmagnesia, lime or limestone, usually necessitate cooling the flue gas toabout 55° C. in order to establish a water phase. At these temperaturesthe treated gas requires reheating in order to develop enough buoyancyto obtain an adequate plume rise from the stack. Moreover, suchprocesses create products involving a solid waste disposal problem.

Various solid phase processes for the removal of sulfur dioxide whichhave hitherto been proposed also have disadvantages. The use oflimestone or dolomite, for example, to adsorb sulfur dioxide creates awaste disposal problem because the solid is not regenerated.

Processes for the removal of sulfur oxides from gases using varioussorbents are discussed in the following: U.S. Pat. No. 2,992,884 toBienstock et al (sorbent: alkali metal oxide dispersed on a carrier suchas alumina or chromia); U.S. Pat. No. 3,411,865 to Pijpers et al(sorbent: alkali metal oxide and iron oxide dispersed on a carrier suchas alumina, magnesia or chromia); U.S. Pat. Nos. 3,492,083 and 3,669,617to Lowicki et al (sorbent: oxide, hydrated oxide or hydroxide ofaluminium, zinc, iron or manganese and an oxide or hydroxide of analkali metal or alkaline earth metal); U.S. Pat. No. 3,589,863 to Frevel(porous alkali metal bicarbonate aggregates); U.S. Pat. No. 3,755,535 toNaber (sorbent: activated alumina or magnesia impregnated on inertcarrier); U.S. Pat. No. 3,948,809 to Normal et al (sorbent: bauxite andalkali metal carbonate); U.S. Pat. No. 3,959,952 to Naber et al(sorbent: alumina carrier impregnated with copper and aluminum,magnesium, titanium or zirconium) and United Kingdom No. 1,154,009(sorbent: vanadium compound and an alkali metal compound).

U.S. Pat. No. 3,880,618 to McCrea et al concern the simultaneous removalof sulfur and nitrogen oxides from gases using alkalized alumina oralkali metal carbonate or oxide.

U.S. Pat. No. 4,071,436 to Blanton, Jr. et al describes the removal ofsulfur oxides using reactive alumina.

Alkalized alumina is discussed in the following: D. Beinstock, J. H.Fields and J. G. Myers, "Process Development in Removing Sulfur Dioxidefrom Hot Flue Gases" 1. Bench-Scale Experimentation, Report ofInvestigations 5735, United States Department of the Interior, pp. 8-17;U.S. Pat. No. 3,551,093 to J. G. Myers et al and U.S. Pat. No. 3,557,025to Emerson et al. As discussed hereinbelow in greater detail, alkalizedalumina sorbents, heretofore utilized for flue gas treatment haveexhibited severe degradation of their attrition resistance due to thechemical processes of adsorption and regeneration.

The alkalized alumina sorbent is manufactured by precipitating dawsonite(NaAl(OH)₂ CO₃) from a solution of Al(SO₄)₃ and Na₂ CO₃ at 90° C. Theresulting solid is then heated to 130° C. to dry the residue moistureand crushed to a small size. Since the dawsonite is formed throughprecipitation, it has a very tight solid structure with little room toabsorb SO₂. Therefore, the chemically bonded H₂ O and CO₂ have to beremoved through calcination at high temperatures in order to form aporous sorbent. ##STR1##

The calcinated sorbent (NaAlO₂), known as alkalized alumina, isthereafter useable in a flue gas treatment process.

Sodium is an integral part of the whole crystal structure of alkalizedalumina. The concentration of sodium in alkalized alumina is about 25%by weight.

The chemical process of adsorption produces changes in the sorbent andcreates internal forces that cause sorbents of a type similar to thoseof the present invention, e.g., alkalized alumina sorbent, to attrite(crumble) rapidly. The sorbents of the present invention do not sufferfrom this attrition problem which has been associated with sorbents of asimilar type, such as alkalized alumina.

As adsorption proceeds, the sulfite/sulfate product layer growth takesplace in both directions from the initial pore boundary, however, thegrowth into the substrate material is limited to only a very thin layerfor the impregnated sorbent. As the product layer grows into thealkalized alumina material itself, it disrupts and distorts the crystalstructure. The product molecule (Na₂ SO₃ and Na₂ SO₄) volumes are muchlarger than the unreacted molecules (Na₂ O) so the product layerproduces a very disturbed and weakened material. As the growthcontinues, the product layer buckles and cracks producing pathways evendeeper into the substrate body. The effect of this process is to createphysical stresses that dramatically increase sorbent attrition. Thegrowth proceeds with both impregnated and coprecipitated sorbent untilall the sodium is consumed or until all the void space within the poreis occupied. Most of the surface area, and consequently the sodium,exists in the many very small pores of the impregnated sorbent. Thedimension of these pores decreases continuously to sizes orders ofmagnitude smaller than the average pore diameter. In fact, many of thepores are of the size of the product molecule.

U.S. Pat. Nos. 4,323,544 and 4,426,365 both assigned to the assignee ofthe present invention, concern processes for the removal of nitrogenoxides using a sorbent comprising alumina having a surface area of about20 m² /g and an alkaline component comprising at least one salt of aGroup IA (alkali metal) or Group IIA (alkaline earth metal).

As pointed out above, a major drawback of heretofore used sorbents forremoval of sulfur oxides and/or nitrogen oxides is that such sorbentssuffer from attrition. The sorbents of U.S. Pat. Nos. 4,323,5444,426,365, which are quite effective in removing pollutants from wastegas streams, begin to suffer irreversible attrition at 175° C.Accordingly, it would be quite advantageous to have a sorbent which isnot only effective in removing gaseous pollutants such as sulfur oxidesand nitrogen oxides, but is also able to withstand high temperatureswithout undergoing attrition.

The present invention provides sorbents that do not unduly degrade (doesnot unduly attrite) as a result of chemical use.

The present invention further provides a method of removing nitrogenoxides and, optionally, sulfur oxides, from waste gas streamssimultaneously, in a single process. Moreover, in the present inventionit is possible to treat waste gas streams at temperatures at which thestreams still have adequate buoyancy to obtain good plume rise from thestack. The sorbents of this invention remove NO₂, as well as therelatively inert NO, in an efficient manner.

The present invention also provides for the removal of nitrogen oxidesand sulfur oxides from waste gases (which process produces elementalnitrogen and elemental sulfur) without producing solid waste productwhich would create a disposal problem. The process of the presentinvention utilizes only relatively small quantities of natural gas orother hydrocarbon fuel.

The present invention also provides for the removal of hydrogen sulfide.

SUMMARY OF THE INVENTION

The present invention concerns a sorbent for removal of gaseous nitrogenoxides, sulfur oxides and hydrogen sulfide from waste gas streamscontaining one or more of gaseous nitrogen oxides, sulfur oxides andhydrogen sulfide. The sorbent of the invention includes an aluminasubstrate, preferably a gamma-alumina substrate, and an alkali oralkaline earth compound, i.e., alkali metal (a Group IA metal) or analkaline earth metal (a Group IIA metal). The alkali or alkaline earthmetal is contained in an amount between 50 and 400 μg per m² ofsubstrate and preferably between 100 and 350 μg per m² of substrate, andmost particularly between 150 and 250 μg per m² of substrate.

The process of the present invention comprises contacting a waste gasstream containing oxides of nitrogen and, optionally, oxides of sulfurwith a sorbent comprising alumina and a alkali or alkaline earthcomponent to sorb at least part of the nitrogen oxides and sulfuroxides. The sorbent having the alkali or alkaline component containedtherein in an amount between 50 and 400 μg per m² of substrate andpreferably between 100 and 350 μg per m² and, particularly between about150 and 250 μg per m². The nitrogen- and sulfur-laden sorbent is thenregenerated by heating the sorbent in a reducing atmosphere, e.g.,hydrogen or hydrogen sulfide-containing gas stream, at temperatures upto about 650° C., whereby nitrogen is removed as elemental nitrogen andsulfur is removed as elemental sulfur. Alternatively, regeneration isconducted by heating the sorbent in an inert atmosphere at temperaturesup to about 350° C. to 650° C., whereby the nitrogen oxide is removed asnitric acid, and then contacting the hot sorbent with a reducing agent,whereby the sulfur is removed as elemental sulfur. The sulfur producedin regeneration may be partially used to produce hydrogen sulfide, whilethe remainder of the sulfur is recovered. The regenerated sorbent isthen used for further removal of oxides of sulfur and nitrogen.

The present invention includes a process for removing hydrogen sulfidefrom a gas stream. This process for hydrogen sulfide removal involvescontacting the gas stream containing hydrogen sulfide with the abovedescribed sorbent at temperatures ranging from 300° C. to 650° C.Regeneration of the spent sorbent for such process is conducted usingsteam, whereby the hydrogen sulfide is displaced from the sorbent andremoved from the sorbent surface. The excess steam is subsequentlycondensed producing a stream of high hydrogen sulfide concentration fordirect use or for further processing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a summary of attrition test results comparing a prior sorbentwith sorbents according to the present invention.

FIG. 2 is a plot of surface area (m² /g) versus Na₂ CO₃ content, weight%, demonstrating the effect of sodium loading on the surface area ofgamma alumina substrate.

FIG. 3 is a flow diagram depicting a process of the present inventionfor the simultaneous removal of sulfur oxides and nitrogen oxides fromflue gas.

FIG. 4 is a flow diagram depicting a process according to the presentinvention for the removal of nitrogen oxides from flue gas.

FIG. 5 is a flow diagram depicting a process according to the presentinvention for the removal of hydrogen sulfide from a H₂ S laden gasstream.

FIG. 6 shows the results of a mercury porosity test to determine thepore volume distribution in gamma alumina substrate.

FIG. 7 is a schematic diagram showing macro and micro pores of a sorbentaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Alumina in the present invention means a form of alumina with anextended surface area, usually above about 100 square meters per gram,and often as high as 400 or 500 square meters per gram. For NOx removalalone, surface areas of 20 m² /g are operative. Many methods are knownin the are whereby such forms of alumina may be prepared. For example,high surface area alumina may be precipitated from a sodium aluminatesolution or sol by the addition of an acidic material such as carbondioxide, mineral acid, or an acidic salt such as aluminum sulfate. Othermethods of producing high surface area aluminas involve the dehydrationof aluminum hydroxides such as aluminum hydrate (Al(OH)₃) or bauxite.Activated bauxite is a particularly useful source of alumina for thepresent invention because of its low cost. A further useful source ofhigh-surface-area alumina for purposes of the present invention are theby-products from the hydrolysis of aluminum alkoxides such as aluminumtri-isopropoxide. Such aluminas which are characterized by extremelyhigh purity in terms of the absence of other metallic elements, haverecently become commercially available at relatively low cost.

Gamma-alumina is the preferred form of alumina for the substrate of thesorbent of the present invention.

The alumina substrate of the present invention has pores therein forreceiving the alkali or alkaline earth component. In an embodiment ofthe present invention, the substrate is prepared by adhering individualparticles of gamma-alumina to each other. Such particles having porestherein. Accordingly, such substrate has internal particle pores of acertain small diameter, d₁, and pores between each particle, i.e.,interstices, of a certain small diameter, d₂. The pores d₁ have anaverage pore diameter of between 30 and 400 Angstroms, preferablybetween 60 and 200 Angstroms, and more particularly, between 80 and 100Angstroms. The pores d₂ have an average particle diameter of between 80and 3000 Angstroms, preferably between 100 and 1000 Angstroms, and moreparticularly, between 200 and 500 Angstroms.

The overall pore volume of the alumina substrate of the invention isbetween 0.4 and 0.8 cc/g, preferably between 0.6 and 0.8 cc/g. Thesurface area of the alumina substrate is between 100 m² /g and 500 m²/g.

It is also within the present invention to utilize as a sorbent solelygamma-alumina (having no further component such as sodium). Suchgamma-alumina having a pore volume of between 0.4 and 0.8 cc/g and asurface area between 100 m² /g and 500 m² /g. Such sorbent can beutilized in the processes described herein in the same manner, i.e.,same temperatures and residence times, as sorbents containing an alkalimetal or alkaline earth metal.

The alkali or alkaline earth component of the sorbent of the presentinvention is an alkali metal, i.e., Group IA metal, namely, Li, Na, K,Rb, Cs or Fr, or an alkaline earth metal, i.e., Group IIA metal, namely,Be, Mg, Ca, Sr, Ba or Ra. The preferred components are sodium, potassiumand calcium, with sodium being particularly preferred.

The alkali or alkaline earth component of the sorbent of the presentinvention may be advantageously incorporated as the hydroxide,carbonate, nitrate, acetate, or other soluble salt of a Group IA metal,or of a Group IIA metal.

It will be understood that mixed salts also may be used as i.e., amixture of salts (1) having the same metal but different anion portions,or (2) having the same anion but different metal portions, or (3) havingdifferent metal and anion portions, may be used. For instance, a mixtureof sodium acetate and carbonate, or a mixture of potassium and sodiumcarbonates, or a mixture of potassium acetate and sodium carbonate maybe advantageously employed.

The sorbent according to the present invention can be prepared by the"dry impregnation" technique. The alkali or alkaline earth component,e.g., sodium, is loaded onto the substrate, e.g., gamma-alumina, byspraying the substrate with a solution of a salt of the alkali oralkaline earth component, e.g., a sodium carbonate solution. Theimpregnated sorbent is subsequently heated to dry the residue moisture.It has been found by preparing the sorbent as described below, that thealkali or alkaline earth component in evenly distributed inside thepores of the substrate. The chemical reactions involved in the sorbentpreparation as described above are as follows:

    Na.sub.2 CO.sub.3(s) +Al.sub.2 O.sub.3(s) +H.sub.2 O.sub.(1) →Na.sub.2 CO.sub.3(s) +Al.sub.2 O.sub.3.H.sub.2 O.sub.(s) →NaAlO.sub.2(s) +NaAl(OH).sub.2 CO.sub.3(s)

The dry impregnated sorbent is the final product and can be readily usedin flue gas cleaning processes. In constrast thereto, alkalized aluminasorbent such as developed by the U.S. Bureau of Mines requirescalcination at high temperature before use in such processes.

In further contrast to alkalized alumina wherein the sodium isdistributed throughout the entire solid matrix, in the sorbent of thepresent invention, the alkali or alkaline component is coated only onthe internal surface, i.e., on the porous structure of the substrate.

The sorbent of the present invention can be further characterized inthat the layer of alkali or alkaline earth salt component, e.g., Na₂CO₃, on the surface of the porous structure does not exceedapproximately one molecule thickness.

Without wishing to be bound by any particular theory of operability, itis believed that there is a critical level of alkali or alkaline earthcomponent, e.g., sodium loading per pore volume, surface area ofsubstrate beyond which the physical strength of the sorbent particle isseverely weakened, thus leading to attrition.

FIG. 1 shows a series of bar graphs of percentage weight loss versusnumber of cycles for three sorbents subjected to attrition testing. Eachbar represents an incremental percentage weight loss over a ten minuteperiod in the attrition test.

The sorbents tested had the following physical characteristics:

    ______________________________________                                        Physical Characteristics                                                                     Sorbent A'                                                                              Sorbent B'                                                                              Sorbent C'                                 ______________________________________                                        % Sodium       3.50      3.46      6.55                                       Compact Bulk Density,                                                                        50.0      42.4      46.3                                       lbs./ft.sup.3                                                                 Surface Area, m.sup.2 /gm                                                                    225       222       144                                        Pore Volume by H.sub.2 O,                                                                    0.22      0.78      0.62                                       cc/gm                                                                         ______________________________________                                    

Sorbents B' and C' are according to the present invention; Sorbent A' isa prior art sorbent.

The sorbents were tested in an Accelerated Air Jet Attrition (AAJA) testapparatus for a period of 30 minutes. The AAJA apparatus was developedby W. R. Grace & Co. AAJA tests measure the attrition strength ofsorbents exposed to various operating conditions over a number ofchemical cycles.

A typical AAJA test apparatus is described as follows:

A 50 gram sample of sorbent previously screened to +10, -20 mesh isplaced in an inverted one liter Pyrex, wide mouth (45 mm diameter),Erlenmeyer flask. The flask has a 6.2 cm diameter hole centered in itsbottom which is covered by a 40 mesh screen. The mouth of the flask isfitted with a nylon stopper having a concave bottom roughly 1.1 cm deep.A 1/4" O.D. (1/8" I.D.) stainless steel tube is inserted through a holein the stopper to a point even with the concave bottom and the tubeopening is covered by a small piece of 60 mesh screen. The stopper isheld tight to the flask mouth by means of two rubber "O" rings fittedinto grooves carved into the side of the stopper.

Air is obtained from a pressurized gas cylinder equipped with a pressureregulator. The air passes through 3/8" flexible, "TEFLON" tubing to adrying tube, a valve, a rotometer (0-6.43 ACFM at 21° C., 1 atm.) andinto the 1/4" tube fitted through the stopper inserted into the mouth ofthe inverted Erlenmeyer flask. The flask is supported by a ring clampattached to a stand and placed within a laboratory fume hood. The flaskis levelled on the stand prior to the start of the test.

Sorbent B' was tested at four different adsorption temperatures rangingfrom 107° C. to 290° C. In each case, the sample was subjected to fivecycles of adsorption/regeneration. The first adsorption was performed ina fluidized bed. The sample was then transferred to a fixed bed reactorfor the remainder of the test. A single sample of Sorbent B' was alsotested over 15 cycles of operation at a constant adsorption temperatureof 120° C. From FIG. 1, it can be seen that the attrition rate of thissorbent appears to be unaffected by adsorption temperature or by thenumber of chemical cycles. The percentage weight loss in the second andthird ten minutes of the attrition test on each sample in this serieswas essentially the same, in most cases less than 1 percent of sampleweight.

The apparent differences in percentage weight loss during the first 10minutes of attrition tests on Sorbent B' are believed to be attributableto different methods of sample preparation. All samples in the series,other than the five cycle test at 290° C., were initially prepared byscreening roughly 250 grams of the sorbent in a mechanical shaker for 20minutes and then extracting a 150 gram sample of the 10×20 mesh sizefraction. For the test at 290° C., small quantities of the sorbent werecarefully screened by hand to eliminate all particles smaller than 10×20mesh prior the test. As seen in FIG. 1, the weight loss in the first tenminutes of the test on this sample was considerably less than all othersin the series. It is believed that a significant portion of the loss inthe first ten minutes for all other samples in this series is due to thepresence of particles smaller than 20 mesh in the original sample placedin the fixed bed reactor.

FIG. 1 also shows the results of attrition tests performed on Sorbent A'and Sorbent B'. The attrition rate of Sorbent A' was considerably higherthan Sorbent B' in tests on both the fresh and the cycled material. Thesample of Sorbent A' tested after five cycles at 177° C. yielded aweight loss of 41.5% after the first ten minutes of the attrition test,as compared with a corresponding 1 to 2% loss for all samples of SorbentB' tested. The major difference between these two sorbents is in porevolume; each has roughly the same surface area and sodium loading. Thepore volume of Sorbent B' is more than three times greater than that ofSorbent A'.

Sorbent C' has twice the sodium loading and roughly 65 percent of thesurface area of Sorbent B'. Sorbent C' was also tested after five cyclesat 290° C.

The sorbents according to the present invention (Sorbents B' and C')exhibited very litte percentage weight loss even at temperatures up to290° C. and with 15 cycles (one absorption-one regeneration per cycle).In contrast, the sorbent with the 0.22 cc/g pore volume (Sorbent A')experienced a dramatic increase in attrition rate at 177° C. It isbelieved that reduced pore volume limits the space available for productlayer expansion, thereby resulting in a significant increase in stressin the sorbent particle and an increased attrition rate. The run at 177°C. for Sorbent A' was aborted after the first ten minutes due to theexcessively large initial weight loss.

The effect of sodium loading on the surface area of gamma-aluminasubstrate is illustrated in FIG. 2. When the sodium loading is low, thesodium salt, i.e., Na₂ CO₃, layer is approximately one molecule thick.When the loading increases to the extent that most of the internalsurface is covered, the molecules of Na₂ CO₃ begin to pile up. As aresult, the thickness of the Na₂ CO₃ layer will increase proportional tothe loading. FIG. 2 indicates that when the Na₂ CO₃ loading is belowabout 8%, the surface area remains fairly constant, because it is onlypartially covered with a mono-molecular layer of Na₂ CO₃. However, whenthe Na₂ CO₃ loading increases above 8%, (e.g., 12 to 20%) the thicknessof the Na₂ CO₃ layer rapidly expands. The expansion of the Na₂ CO₃ layerresults in reduced pore diameters and consequently reduced surfaceareas.

A comparison of the typical characteristics of a sorbent according tothe present invention and alkalized alumina is as follows:

    ______________________________________                                                      Sorbent According To                                                                        Alkalized                                                       Present Invention                                                                           Alumina                                           ______________________________________                                        Total Sodium Loading, wt %                                                                    3.5             20-25                                         Surface Area, m.sup.2 /g                                                                      222             47                                            Pore Volume, cc/g                                                                             0.69            0.69                                          Average Pore Diameter, nm                                                                     12.4            58.7                                          ______________________________________                                    

The exact chemical or crystallographic form of the sorbent is notnarrowly critical in the present invention. It is believed that thealumina in the prepared sorbent is poorly crystalline and exists asgamma-Al₂ O₃ and gamma-Al₂ O₃ H₂ O. When sodium is employed as Na₂ CO₃,it exists in the sorbent as Na₂ CO₃, Na₂ CO₃ H₂ O, gamma-NaAlO₂,beta-NaAlO₂ and NaAl(OH)₂ CO₃. The sorbent may change in structure afterregeneration as compared to its fresh condition. The alkaline componentmay be present as the oxide, hydroxide, carbonate, or aluminate, ormixtures of these compounds, when the sorbent is freshly prepared orafter it has been regenerated. Various amounts of sulfur or nitrogencontaining salts may also be present, such as nitrates, nitrite,sulfate, sulfite, or sulfide.

Various other metallic oxides, such as copper, iron, vanadium, zinc,molybdenum, or rare earth elements, may also be present in amounts up toabout 10 atom percent, based on the total atoms of aluminum, alkalinecomponent, and other metal(s).

The waste gas stream containing nitrogen oxides and sulfer oxides iscontacted with the sorbent at temperatures of 85° C. to about 200° C.,and preferably about 90° C. to 150° C. At higher temperatures theefficiency of nitrogen oxide removal is decreased, while at lowertemperatures the waste gas stream would require reheating orrecompression to develop adequate stack plume.

The sorbent and waste gas may be contacted in a fixed bed, fluid bed, ormoving bed, according to methods which are known in the art. If thecontacting is in a fixed bed the gas residence time is in the range of0.1 to about 10 seconds but a wider range is possible in fluid bedoperation.

After the sorbent has become laden with nitrogen and, optionally,sulfur, preferably to a level corresponding to greater than about oneequivalent of nitrogen plus sulfur for each five equivalents of alkalinecomponent, it is regenerated. For this purpose one equivalent of sulfuris taken as one-half of a gram-atom, one equivalent of nitrogen is onegram-atom, one equivalent of alkali metal is one gram-atom, and oneequivalent of alkaline earth metal is one-half of a gram atom. Thesorbent is regenerated by contact with a regenerant gas streamcontaining at least 0.01 atmosphere partial pressure of reducing gassuch as hydrogen or hydrogen sulfide, at temperatures of about 350° C.to about 700° C., for a period of time sufficient to recover asubstantial portion of the sorbent's capacity for nitrogen oxide andsulfur oxide sorption. The minimum time required for regenerationdepends strongly on the temperature and partial pressure of hydrogensulfide in the regenerant gas, and may vary from a few minutes at 750°C. for 12 hours or more at lower temperatures.

The regenerant gas preferably contains carbon dioxide or water vapor,and, more preferably, contains both carbon dioxide and water vapor.Alternatively, the sorbent is treated with carbon dioxide and/or watervapor after contacting with the hydrogen-sulfide containing regenerantgas. When carbon dioxide and/or water vapor are used, such arepreferably employed in total amounts corresponding to at least about onemole of carbon dioxide and/or water vapor per mole of oxide gas sorbedbefore regeneration.

A convenient means of obtaining a suitable regenerant gas containingcarbon dioxide is by the catalytic vapor phase reaction of steam,sulfur, and a hydrocarbon such as methane, essentially according to thefollowing reaction:

    CH.sub.4 +2H.sub.2 O4S→4H.sub.2 S+CO.sub.2.

For the purpose of this invention, the use of hydrogen sulfide in theregenerant gas should be taken to include the use of other compoundswhich will essentially form hydrogen sulfide under the conditions ofregeneration, viz. carbon disulfide or carbon oxysulfide in the presenceof steam, such as by the following reactions:

    CS.sub.2 +2H.sub.2 O⃡CO.sub.2 +2H.sub.2 S,

or

    COS+H.sub.2 O⃡CO.sub.2 +H.sub.2 S.

During regeneration sulfur forms in the regenerant stream and iscondensed by cooling downstream from the sorbent. During this process atleast part of the hydrogen sulfide is converted to elemental sulfur. Anyunconverted hydrogen sulfide can be readily recycled after the sulfurhas been condensed.

After the sorbent has been regenerated, it is cooled to the sorptiontemperature, for example, by contacting with a cooler water gas stream.The sorbent is then re-used for removing sulfur oxides and nitrogenoxides.

In an embodiment of the present invention, a concentrated stream ofnitrogen oxides removed by a sorbent, such as the sorbent describedherein, from waste gas streams containing the same can be recycled backto the source of such nitrogen oxides, e.g., a steam boiler, afterregenerating the sorbent, i.e., driving off the nitrogen oxides from thesorbent. In this embodiment, the nitrogen oxide level in the boilerreaches a certain equilibrium, e.g., 600 ppm, and thus the recyclednitrogen oxides will be broken down in the steam boiler withoutincreasing the nitrogen oxides ppm in the stack effluent gas.

Referring to FIG. 3, wherein like numerals indicate like elements, thereis shown a flue gas stream 12 containing both SO_(x) and NO_(x) from apower plant (not shown) which is passed through a fluid bed adsorber 14containing sorbent according to the present invention. Adsorber 14 has afluidizing grid 15. The sulfur oxides and nitrogen oxides are adsorbedon the surface of the sorbent and removed from the flue gas stream.

The saturated sorbent 16 is subsequently transported to a staged, fluidbed heater 18 wherein the sorbent temperature is raised above 532° C.(1000° F.) using high temperature air 20 supplied by air heater 22 whicha stream of ambient air 24 and a fuel stream 26, e.g., natural gas,enter. Simultaneously, the sorbed NO_(x) is stripped from the sorbentand carried away in the hot gas stream which passes through cylone 28and via stream 30 is mixed with the power plant combustion air stream(not shown).

The hot sorbent is removed from the sorbent heater 18 into a moving bedregenerator 32 via line 34. In the moving bed regenerator 32, thesorbent is contacted with a regenerant gas stream 36. The regenerant gas36 reacts with the sorbed sulfur oxides to produce elemental sulfur.Off-gas stream 38 containing elemental sulfur is transported into asulfur condenser and mist eliminator 45 wherein a steam stream 42, waterstream 44 and elemental sulfur stream 46 are produced. A stream 40 fromsulfur condenser and mist eliminator 45 is returned to regenerator 32.

The regenerated sorbent is transported via stream 48 past valve 50 to astaged, fluid bed sorbent cooler 52, where it is contacted withatmospheric air supplied via line 54 to reduce its temperature to about120° C. (250° F.). The heated atmospheric air 56 subsequently istransported to gas heater 22 where its temperature is increased wellabove 532° C. (1000° F.) for use as the heated medium in fluid bedheater 18.

Cooled sorbent via line 58 is transported by air in line 54 to apneumatic lift line 60 into cyclone separator 62 via stream 64. Cycloneseparator 62 separates stream 64 into air 66 and sorbent 68. Sorbentstream 68 enters adsorber 14. The discharge gas from adsorber 14 exitsvia line 70.

In FIG. 4, a flue gas stream 72 containing NO_(x) from a combustionfacility (not shown) which uses a fuel free of sulfur is passed througha fluidized bed sorber 74 containing sorbent and having a fluidizinggrid 76. The NO_(x) free sorbent is returned to the sorber 74 for reusevia line 108 and the air that was entrained therewith is separated bycyclone 104 and exits via line 110. The NO_(x) is removed by sorption onthe surface of the sorbent. The NO_(x) free flue gas (clean gas stream)is subsequently discharged to the atmosphere via line 78.

As the sorbent becomes saturated with NO_(x), it is transported via line80 to a staged, fluid bed heater 82, wherein it is contacted with astream of hot air 84. The hot air 84 is generated by an air heated 86wherein fuel 88 for example, natural gas and ambient air 90 enter. Thehot air strips the sorbent NO_(x) from the surface sorbent. The hot gasstream containing NO_(x) exits heater 82 via line 92 and is directed toa cyclone 94. Air 92 from cyclone 94 is used in the combustion facility(not shown). The NO_(x) free sorbent stream 98 from fluid bed heater 82is then transported via a pneumatic lift 100 by air supplied by line 102to cyclone 104 via line 106 according to the present invention.

In FIG. 5, a gas stream 112 containing hydrogen sulfide as acontaminant, as from, for example, a oil shale retort or a coalgasification unit is passed through a fluidized bed sorber 114containing sorbent according to the present invention. The sorbent restson grid 116 in sorber 114. Hydrogen sulfide is removed from the gasstream by sorption on the surface of the sorbent. The cleaned gas streamleaving the sorber 114 via line 118 is used for its intended purpose. Ahydrogen sulfide saturated sorbent stream 120 from sorber 114 istransported into a fluidized bed regenerator 122 where it is contactedwith steam stream 124. The steam strips the hydrogen sulfide from thesorbent suface. In regenerator 122, the sorbent rests on grid 126. Asteam and hydrogen sulfide stream 128 from regenerator 122 is thentransported into a water condenser 130, where the excess steam isremoved producing a high concentration hydrogen sulfide gas stream 132and a water stream 134. The stripped sorbent from regenerator 122 isreturned to sorber 114 via line 136 for reuse.

FIG. 6 shows the results of a mercury porosity test to determine thepore volume distribution in the gamma-alumina substrate of the sorbentof the present invention. "D" in FIG. 6 refers to diameter. Thedistribution is bimodal and may be separated into a group of microporeshaving a probable pore radius ranging from 10 to 100 Angstroms, and agroup of macropores with radii at least one and possibly two orders ofmagnitude larger than the micropore radii.

FIG. 7 is a schematic diagram showing macro and micro pores in thesubstrate of the sorbent of the present invention. The solid circlesconsist of tightly packed uniform spherical grains. The macroporescorrespond to the space in the interstices between grains, while themicropores correspond to the pores within each grain. A greater portionof the sorbent's surface area resides in the micropores, but a greaterportion of pore volume is in the macropores. It is believed that themicropores, with high surface area, are where most of the chemicalreaction takes place. The size of the micropores, 10 to 100 Angstroms,is similar to the size of the molecules produced on adsorption of SO₂.As SO₂ is adsorbed, the products expand into the void space available inthe pore. A mechanical stress may be placed on the solid structure underconditions of high SO₂ loading and/or low micropore volume. The stressresults in an unacceptably high rate of sorbent attrition.

The invention will now be described with reference to the followingnon-limiting examples:

EXAMPLE 1

Gamma-alumina prepared by the Davison Division of W. R. Grace & Co.according to U.S. Pat. Nos. 4,154,812 and 4,279,779 was treated with asurface coating of Na₂ CO₃ equivalent to 8 weight percent Na₂ CO₃. Toaccomplish this, the required amount of Na₂ CO₃ was dissolved in asolution which was applied to the surface via an incipient wetnesstechnique. The solid was then heated to drive off moisture, leaving alayer of Na₂ CO₃ on the surface. The physical properties of theresulting sorbent were as follows:

weight percent sodium: 3.5

N₂ surface area, m² /g: 222

Pore volume by H₂ O, cc/g: 0.78

Compacted bulk density, g/cc: 0.68

EXAMPLE 2

A 100 g sample of sorbent prepared according to Example 1 (onlygamma-alumina) was placed in a 2 foot by 2 inch diameter fixed bedreactor heated by a temperature-controlled tube furnace. The sorbent wascontacted at 120° C. with actual flue gas from a coal-fired boilerhaving the following approximate volume composition: 74% N₂, 12% CO₂, 9%H₂ O, 4% O₂, 0.23% SO₂, 0.05% NO, and 0.0025% NO₂. The flue gas flowratewas 10 liters per minute, measured at 25° C. and 1 atmosphere pressure.

Samples of the reactor effluent were analyzed for nitrogen oxides(NO+NO₂, expressed as NO_(x)) and for sulfur dioxide with the followingresults:

    ______________________________________                                        Time On Stream                                                                (minutes)      % NO.sub.x Removal*                                            ______________________________________                                        15             100                                                            30             98                                                             45             95                                                             60             90                                                             75             80                                                             90             58                                                              6             100                                                            12             100                                                            18             100                                                            24             95                                                             30             82                                                             45             40                                                             ______________________________________                                         ##STR2##                                                                 

EXAMPLE 3

A 100 g sample of sorbent prepared according to Example 1 (containingweight 8% Na₂ CO₃) was contacted with actual flue gas, using the sameoperating conditions and feed stream composition as in Example 2. Theresults were as follows:

    ______________________________________                                        Time On Stream                                                                (minutes)                                                                     ______________________________________                                                       % NO.sub.x Removal*                                            15             100                                                            30             97                                                             45             94                                                             60             94                                                             75             93                                                             90             88                                                             105            65                                                                            % SO.sub.2 Removal*                                            30             100                                                            60             94                                                             90             83                                                             120            57                                                             150            42                                                             ______________________________________                                    

By comparing the results of Example 2 (only gamma-alumina) with theresults of Example 3 (gamma-alumina plus 8% Na₂ CO₃), it is clearly seenthat with increasing time on stream, the sorbent of Example 3 resultedin better removal of NO_(x) and SO_(x).

EXAMPLE 4

A 100 g sample of sorbent prepared according to Example 1 (containingweight 8% Na₂ CO₃) was heated to 570° C. and contacted with a gascontaining 30% H₂ S and 70% N₂ by volume. The gas flow rate was 60liters per hour, measured at 25° C. and 1 atmosphere pressure. Samplesof the reactor effluent were continuously analyzed for H₂ S with thefollowing results:

    ______________________________________                                        Time On Stream                                                                (minutes)      % H.sub.2 S Removal*                                           ______________________________________                                        2.5            100                                                            5              100                                                            7.5            100                                                            10             90                                                             12.5           60                                                             15             40                                                             ______________________________________                                         ##STR3##                                                                 

EXAMPLE 5

The spent sorbent of Example 3 was regenerated by heating to 550° C. inN₂ gas, and then introducing a gas stream containing 30% H₂ S in N₂ for50 minutes at 60 liters per hour. The sorbent was then contacted withsteam and cooled to 120° C. After a second sorption cycle similar toExample 3, the results were as follows:

    ______________________________________                                        Time On Stream                                                                (minutes)                                                                     ______________________________________                                                       % No.sub.x Removal                                             15             100                                                            30             98                                                             45             96                                                             60             95                                                             75             94                                                             90             88                                                             105            63                                                                            % So.sub.x Removal                                              0             100                                                            60             96                                                             90             70                                                             120            35                                                             ______________________________________                                    

Similar results were obtained after this sample had beenadsorbed/regenerated a total of 15 times.

EXAMPLE 6

Two samples of sorbent were subjected to five cycles ofadsorption/regeneration and then tested in an Air Jet AttributionApparatus (AJAA) to determine if there was any change in attritionproperties with chemical cycling. The physical properties of the twosorbents, prior to chemical cycling, were as follows:

    ______________________________________                                        SORBENT           Sorbent A Sorbent B                                         ______________________________________                                        Sodium content, wt. %                                                                           4.4       3.5                                               N.sub.2 surface area, m.sup.2 /g                                                                225       222                                               Pore volume by H.sub.2 O, cc/g                                                                  0.22      0.78                                              Compacted bulk density, g/cc                                                                    0.8       0.68                                              ______________________________________                                    

The sorbents used in Example 6 were made from different substrates, butthe method of depositing the Na₂ CO₃ on the surface of the substrate wasthe same in each case. Electron micrographs showed the surface of theSorbent B using a gamma-alumina substrate supplied by the DavisonDivision of W. R. Grace & Co. to consist of similarly shaped, platelikestructures with large areas of common bonding. Examination of thesurface of the Sorbent A using a substrate of Reynold's gamma-aluminawas depicted by electron micrograph to consist of tiny irregularlyshaped particles with small areas of common bonding.

Sorbents A and B were placed in 2 foot by 2 inch diameter fixed bedreactor heated by a temperature-controlled tube furnace. Sorbents A andB were contacted at a temperature of 177° C. with actual flue gas from acoal-fired boiler containing, on average, 74% N₂, 12% CO₂, 9% H₂ O, 4%O₂, 0.21% SO₂, 0.05% NO, and 0.0025% NO₂. The flue gas flowrate in eachcase was 10.0 liters per minute, measured at 25° C. and 1 atmospherepressure. Sorbents A and B were then regenerated by heating the fixedbed reactor to 550° C. in N₂, and then introducing a stream containing30% H₂ S and 70% N₂ at 60 liters per hour, measured at 25° C. and 1atmosphere pressure. After regeneration, Sorbents A and B were contactedwith steam, cooled to 120° C., and then reused in the adsorptionprocess.

After five cycles of adsorption, a 50 g sample of Sorbent A and SorbentB were extracted and tested for attrition strength in an Air JetAttrition Apparatus (AJAA). The results of this test were as follows:

    ______________________________________                                                                               AJAA                                                    Adsorption            Weight                                         No. Of   Temperature Length of Test                                                                          Loss                                   Sorbent Cycles   (°C.)                                                                              (minutes) (%)                                    ______________________________________                                        Sorbent A                                                                             Fresh    NA          30        24.3                                           5 Cycles 177         10        41.5                                   Sorbent B                                                                             Fresh    NA          30        4.3                                            5 Cycles 177         30        3.0                                    ______________________________________                                    

Attrition losses measured in this test for fresh (unreacted) Sorbent Awere over five times greater than fresh Sorbent B. In addition, theperformance of Sorbent A deteriorated substantially with chemicalcycling, while that of Sorbent B did not. Sorbent loading was similarfor Sorbents A and B, corresponding to 5 grams SO₂ adsorbed per 100grams sorbent. The relatively poor performance of Sorbent A vis-a-visSorbent B is believed to be attributed to differences in the surfacestructure of the two substrates.

EXAMPLE 7

Sorbent B used in Example 6 was tested to determine if the attritionrate would be affected by increases in the temperature of adsorption ornumber of cycles of adsorption and regeneration. The methods ofadsorption and regeneration were the same as that described in Examples3 and 5. The average sorbent load varied with adsorption temperature,ranging from 5 to 2.5 grams SO₂ adsorbed per 100 grams sorbent, at 88°C. and 300° C., respectively. The sorbent load in a series of 5, 10 and15 cycle tests at 120° C. was constant at 5 grams SO₂ adsorbed per 100grams sorbent. The results were as follows:

    ______________________________________                                                                           AJAA                                                 Adsorption               Weight                                     No. Of    Temperature  Length of Test                                                                            Loss                                       Cycles    (°C.) (minutes)   (%)                                        ______________________________________                                        Fresh     NA           30          4.3                                        5 Cycles  105          30          3.9                                        5 Cycles  120          30          3.2                                        5 Cycles  177          30          3.0                                        5 Cycles  285          30          1.7                                        10 Cycles 120          30          3.8                                        15 Cycles 120          30          5.2                                        ______________________________________                                    

The above results show that the attrition rate of Sorbent B isunaffected by increased temperature of adsorption or increased number ofchemical cycles.

EXAMPLE 8

The AJAA test is commonly used in industry to assess the relativeattrition strengths of different solid catalysts. To predict attritionlosses in large scale processing, however, the test data must becorrelated with actual measured attrition losses in pilot or commercialscale systems. Samples of fresh (unreacted) Sorbent B were submitted tothe United States Department of Energy ("DOE") for testing on their AJAAapparatus. The results were as follows:

    ______________________________________                                                     Cumulative                                                                    % Weight Loss                                                                             Cumulative                                           Time of Test (AJAA as used                                                                             % Weight Loss                                        (minutes)    In Example 6)                                                                             (DOE AJAA)                                           ______________________________________                                        10           2.6         0.12                                                 20           3.8         0.27                                                 30           4.2         0.49                                                 ______________________________________                                    

Obviously, the AJAA according to Example 6 subjects the sample to a moresevere test of attrition strength. Such differences are not uncommon inAJAA testing where the results are very sensitive to the design of thetest apparatus, i.e., the air nozzle design (small differences in theair nozzle design can cause differences in the test results).Nevertheless, all results reported in Examples 6, 7, and 9 (below) weregenerated through identical procedures using the same test apparatus.The data may therefore be compared to assess the relative attritioncharacteristics of the samples tested.

The Department of Energy reports AJAA test losses similar to those citedabove for fresh Sorbent B have been obtained on samples from batchesthat exhibited a steady-state attrition rate of 0.02 to 0.03% of thecirculating solids inventory measured in pilot-scale tests on afluidized bed adsorber.

EXAMPLE 9

A sample of Sorbent B was coated with Na₂ CO₃ equivalent to 21 weight %.The physical properties of this sorbent, designated Sorbent C, are asgiven hereinbelow. The physical properties of the substrate and SorbentB are shown for comparison. Note that Sorbent C has considerably lesssurface area and pore volume than either the substrate or Sorbent B.

    ______________________________________                                                       Substrate                                                                     (gamma-                                                                              Sorbent                                                                alumina)                                                                             B        Sorbent C                                      ______________________________________                                        % sodium         0        3.5      9.1                                        N.sub.2 surface area, m.sup.2 /g                                                               216      222      122                                        Pore volume by H.sub.2 O, cc/g                                                                 0.78     0.78     0.48                                       Compacted bulk density, g/cc                                                                   0.62     0.68     0.86                                       ______________________________________                                    

Sorbent C was subjected to five cycles of adsorption in flue gas at 260°C. in a procedure identical to that of Example 3. Sorbent C wasregenerated by heating the reactor to 650° C. in N₂, then introducing astream containing 30% H₂ and 70% N₂ at about 1 liter per minute for 40minutes. The results of this test are given hereinbelow. An identicalseries of tests using H₂ S as the reducing gas is shown for comparison.It should be noted that treatment with H₂ S does not fully regeneratethe sorbent. Furthermore, as SO₂ loading increases on regeneration withH₂, loss on attrition increases substantially.

    ______________________________________                                        Regenerant          H.sub.2                                                                              H.sub.2 S                                          ______________________________________                                        Number of cycles    5      5                                                  Average SO.sub.2 loading*                                                                         13     6                                                  Time of test (minutes)                                                                            20     30                                                 AJAA weight loss (%)                                                                              19.4   5.0                                                ______________________________________                                         ##STR4##                                                                 

The unacceptably high rate of sorbent attrition exhibited by Sorbent Cis believed to be caused by a combination of low surface area and highSO₂ loading. When Sorbent B with twice the surface area and 40% of theSO₂ loading shown above was subjected to a similar test (see Example 8),measured attrition loss was 2 to 5 weight percent in a 30 minute test.Sorbent surface area, pore volume, and SO₂ loading are critical factorsin obtaining economically acceptable rates of sorbent attrition.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

What is claimed is:
 1. A regenerable attrition resistant sorbent usefulin a fluidized bed for the removal of gaseous nitrogen oxides, sulfuroxides and hydrogen sulfide from waste gas streams, which sorbentcomprises(a) a gamma alumina substrate having a surface area between 100m² /g and 500 m² /g and a pore volume between 0.4 and 0.8 cc/g, saidgamma alumina substrate having a bimodal pore size distributioncomprising micropores and macropores, said micropores having an averagepore diameter d₁ in the range of between 30 and 400 Angstroms and saidmacropores having an average pore diameter d₂ in the range of between 80and 3000 Angstroms, and (b) an alkali metal component, said substrateimpregnated with said alkali metal component and the amount of saidalkali metal component being between 50 and 400 μg per m² of saidsubstrate.
 2. The sorbent of claim 1, wherein said amount of said alkalimetal component is between 100 and 350 μg per m² of said substrate. 3.The sorbent of claim 1, wherein said amount of said alkali metalcomponent is between 150 and 250 μg per m² of said substrate.
 4. Thesorbent of claim 1, wherein said macropores of said substrate have anaverage pore diameter d₂ of 100 and 1000 Angstroms.
 5. The sorbent ofclaim 1, wherein said macropores of said substrate have an average porediameter d₂ of 200 and 500 Angstroms.
 6. The sorbent of claim 1 whereinsaid micropores have an average pore diameter d₁ of between 60 and 200Angstroms.
 7. The sorbent of claim 1 wherein said micropores have anaverge pore diameter d₁ of between 80 and 100 Angstroms.
 8. The sorbentof claims 1 wherein said pore volume is between 0.6 and 0.8 cc/g.
 9. Thesorbent of claim 1, wherein said alkali metal component is selected fromthe group consisting of sodium, lithium, potassium, rubidium, cesium andfrancium.
 10. The sorbent of claim 9, wherein said alkali metalcomponent is sodium.
 11. The sorbent of claim 9 wherein said alkalimetal component is potassium.
 12. A regenerable attrition resistantsorbent useful in a fluidized bed for the removal of gaseous nitrogenoxides, sulfur oxides and hydrogen sulfide from waste gas streams, whichsorbent comprises:(a) a gamma alumina substrate having a surface areabetween 100 m² /g and 500 m² /g and a pore volume between 0.6 and 0.8cc/g, and (b) an alkali metal component, said substrate impregnated withsaid alkali metal component and the amount of said alkali metalcomponent being between 150 and 250 μg per m² of said substrate.
 13. Thesorbent of claim 12, wherein said alkali metal component is selectedfrom the group consisting of sodium, lithium, potassium, rubidium,cesium and francium.